Summer Triathlons while Marathon Training

Haven't done this, but I did do a decent amount of biking (no swimming though) to supplement while training for Chicago '08 and '09. It definitely helped me ease back in and keep strong while coming back from an injury, and even the next summer, when I was healthy, and averaged 80+mpw (high of 104), I could bike a decent amount and it definitely did not hurt. Not sure it helped, but it didn't hurt.

It might be a little worse if you're trying to bike enough to be serious about triathlons, but if you're just doing enough to be able to put in a good effort in a sprint tri (since you have no bike, this is probably about where you're at), you'll be fine.

Do this a few days a week: Run a serious run in the morning to get your real mileage in. After work, go bike for an hour, and then do an easy 3-4 mile run. This gets your training in for both, allows your biking to supplement your running, and gets the 'brick' effect of getting used to running off the bike. Add a swim a couple times a week on the days you don't bike, and you've got yourself set up well to find out if you really want to be a triathlete or not. If you like it that much, you'll have figured it where to take it from there.